CX3CR1 and pulmonary fibrosis: In contrast, in the murine model of bleomycin-induced pulmonary fibrosis, an up-regulation of the CX3CL1-CX3CR1 axis was associated with intrapulmonary accumulation of pro-fibrotic M2 macrophages, while CX3CR1-/- mice exhibited a reduction of M2 macrophage recruitment and collagen production 16, suggesting a pro-fibrotic role of the CX3CL1-CX3CR1 axis.